1. Field of the Invention
The invention relates to mild, surfactant-containing compositions in the form of microemulsions for cleansing the skin and for cleaning hard surfaces.
2. Description of the Related Art
While the use of macroemulsions as cleaning compositions for virtually all surfaces has long been known, corresponding microemulsions have been described only recently. In contrast to thermodynamically metastable macroemulsions, microemulsions are equilibrium systems, and are therefore completely stable. Based on the high level of fine division of the system, its free energy is negative as a result of the strong increase in dispersion entropy, despite the infinite, although very small, surface tension, and the system is therefore stable. A consequence of the fine division--diameters of around 10 nm (5 to 100 nm) are typical as opposed to &gt;1 .mu.m in the case of macroemulsions--is the transparency or translucency of the liquids. Microemulsions therefore often give the impression of being true solutions. They consist of the two mutually immiscible liquids, generally water and an apolar liquid, of the surfactant or surfactant mixture, and usually of a cosolvent, occasionally also referred to as cosurfactant. Typical cosolvents are lower alcohols of 3 to 5 carbon atoms. Their effect is firstly to reduce the surface tension between the two liquids, as well as that of the surfactant, and additionally to model the interfacial film (cf. D. Langevin, Mol. Cryst. Liquid Cryst. 1986, 138, 259-305).
Microemulsions as cleaning systems have been described since the mid-1980s. For instance, the patent applications EP-A-0 137 616 and EP-A-0 160 762 claim liquid heavy-duty detergents in the form of an O/W microemulsion consisting of terpenes, paraffins, alkyl-aromatics and/or halogenated hydrocarbons as fat-dissolving solvents, water, customary surfactants, especially based on petrochemicals; electrolytes and amines as stabilizers. Cleaners for hard surfaces that are in the form of microemulsions are claimed by EP-A-0 316 726, EP-A-0 368 146, EP-A-0 620 271 and DE-A-37 16 526. They comprise customary surfactants, cosolvents such as water-soluble alcohols, polypropylene glycols, monoalkyl ethers of glycols, aliphatic carboxylic acids or phosphoric esters, in addition to fragrances and/or hydrocarbons as apolar liquids and, if desired, electrolyte. Finally, the documents EP-A-0 478 086 and WO 92/03528 describe specific cleaners having disinfecting and abrasive properties, respectively, for hard surfaces, that are in the form of microemulsions. They are based on customary, predominantly petrochemical surfactants and customary cosolvents, the latter document making mention, alongside many other types of surfactant, of alkylpolyglucosides.
The use of alkylpolyglucosides as a basic surfactant in microemulsions has been known for some years (cf. H. Luders and D. Balzer, Proc. 2nd World Surfactant Congr. 1988, Paris, Vol. II, 81-93). Nevertheless, this property, unlike that of, for example, fatty alcohol oxethylates, was always associated with the use of customary cosolvents, such as C.sub.3 -C.sub.6 alcohols, alkylglycols, alkyloligoglycols, amines, etc., all of which are unacceptable in odor and/or objectionable on toxicological grounds, so that there is virtually no question of using these microemulsion systems as manual cleaning liquids, let alone in cosmetic applications. In the context of the latter, however, alkylpolyglucosides are of particular interest. Thus, human toxicological tests (DKT) have shown that these compounds are particularly skin-friendly in comparison with other surfactants widely used in cosmetology. The reason for this may lie in the lower degreasing effect in comparison with, say, fatty alcohol ether sulfates, as has been shown in degreasing tests on pigskin loaded with radiolabeled sebum.
This greater skin mildness is also achieved in the case of mixtures of C.sub.10 C.sub.12 -and/or C.sub.12 C.sub.14 -alkylpolyglucosides with ether sulfates, as shown by the much higher residual radioactivity in relation to pure ether sulfate. Consequently, it should also be possible to use alkylpolyglycosides in the form of aqueous microemulsions for cosmetic applications in particular. Similar comments apply to all cleansing processes where the surface of the skin may come into contact with the cleansing liquid. Implicit in this objective, however, is that it is possible to find suitable cosolvents which do not have the above-mentioned disadvantages but are toxicologically unobjectionable and at least neutral in terms of odor.